When a metal tube does not satisfy any requirement in quality, strength, or dimensional accuracy in a hot finished condition, the metal tube needs to be subjected to a cold working process. Such cold working is generally performed by a cold drawing method using a die and a plug or mandrel, or a cold rolling method by a cold pilger mill.
In a conventional cold rolling method by a cold pilger mill, a hollow shell is subjected to a diameter reducing rolling between a pair of rolls having a tapered groove whose diameter gradually decreases in a circumferential direction, and a tapered mandrel whose diameter gradually decreases similarly in a longitudinal direction. That is, each roll of the pair is provided with a groove on its circumference and the groove is shaped such that its width is narrowed as the rolling occur. The rolls repeatedly move forward and backward while rotating along the taper of the mandrel, thereby rolling the hollow shell between the roll and the mandrel (such as Non Patent Literature 1).
FIG. 1 is a diagram to show a rolling mechanism by a conventional cold pilger mill, in which FIG. 1A illustrates a starting point of forward stroke, and FIG. 1B illustrates a starting point of backward stroke. A roll housing having a pair of grooved rolls 2 makes a reciprocating movement via a connecting rod of a crank mechanism. At that moment, pinions which are integrated with the rolls 2 engage with racks so that the rolls 2 are caused to rotate in association with the reciprocating movement.
The cold pilger mill includes a pair of grooved rolls 2 and a mandrel 4. The grooved roll 2 has in its outer circumference a groove whose diameter smoothly varies from an outer diameter (d0 in the diagram) of the hollow shell 1 to the outer diameter (d in the diagram) of a finished rolled tube 5 as being from the engaging entry side toward the finishing exit side of the roll. Further, the mandrel 4 also has a tapered shape whose diameter smoothly varies in a similar fashion. Thus, the roll housing having the above described rolls 2 makes a reciprocating movement thereby rolling the tube material (hollow shell) 1.
The tube material 1 is given a predetermined amount of travel (feed) and rotation (turn) angle immediately before the start of a forward stroke. In a normal rolling, the feed is about 5 to 18 mm and the turn angle is about 60°. In this arrangement, the tube material is subjected to a diameter reducing rolling in both the forward and backward strokes. It was not, however, possible until about 25 years ago to give a feed and a turn angle to the tube material in the backward stroke, and only a re-rolling is performed to remove an elastic restitution in the elongation rolling of forward stroke.
Meanwhile, in around 1985, a drastic simplification of the overall structure of the cold pilger mill was achieved by replacing its mechanical interlocking mechanism with a mechatronics system. That is, the structure of the facility is simplified and its size is reduced by replacing the mechanical intermittent motion with an electric driving mechanism. Particularly, adopting electric control and hydraulic servo control has eliminated complex works such as the replacement of cams and allowed the setting of the feed amount and turn angle of the tube material to be performed simply and precisely in a stepless manner so that the setting and change of the turn angle and feed can be performed with the simple touch of a button.
In this way, there was produced a surplus in the driving energy that had been primarily consumed for the purpose of the transmission of power, which makes it possible to give certain amounts of feed and turn angle even immediately before the start of a backward stroke, however, it turned out that when a little feed is given, yet an excessive rolling load is generated, while not so much in giving a turn angle, resulting in the overloading of the facility, thereby disabling the rolling. Of course, an imbalance in the load between both forward and backward strokes would become pronounced.